[flang][hlfir] fix elemental subroutine calls

genElementalCall can return a null option when lowering elemental
subroutine calls (as there is no return value). Therefore
std::option::value should not be used as it will cause an
assertion failure.

This fixes uses of the mvbits intrinsic with array arguments, as used in
the gfortran test suite.

Differential Revision: https://reviews.llvm.org/D154340